Recent advancements in all-inorganic and organic-inorganic hybrid metal halide materials for photocatalytic CO₂ reduction reaction

Abstract

The utilization of solar energy to address energy and environmental challenges has a seen a significant growth in recent years. Metal halides, which offer unique advantages such as tunable bandgaps, high light absorption efficiencies, favorable product release rates, and low exciton binding energies, have emerged as excellent photocatalysts for energy conversion. This paper reviews the recent advancements in both all-inorganic and organic-inorganic hybrid metal halide photocatalytic materials, including the fundamental mechanisms of photocatalytic CO₂ reduction, various synthesis strategies for metal halide photocatalysts, and their applications in the field of photocatalysis. Finally, it examines the current challenges associated with metal halide materials and explores potential solutions for metal halide materials, along with their future prospects in photocatalysis applications.

Graphical abstract

Keywords

metal halide / photocatalysts / organic-inorganic hybrid / all-inorganic / photocatalytic CO₂ reduction

1 Introduction

In today’s world, rapid development of industry and growing global population [1–3] have led to increased exploitation and excessive consumption of fossil fuels [4], resulting in an energy shortage crisis and worrying global environmental situation [5]. The issue of the greenhouse effect, particularly the excessive emission of carbon dioxide (CO₂) and its slow conversion, is gaining increasing attention from the scientific community. This poses a significant threat to ecosystems, making it urgent to address and resolve this problem [6]. It is imperative to develop sustainable and environmentally friendly technologies [7–9], to achieve the goals of Sustainable Development Strategy, such as “carbon peak” and “carbon neutrality” in the future [10].

Fortunately, inspired by the photosynthetic behavior of plants, it is undoubtedly a wise choice to transform CO₂ into valuable materials such as methane, ethanol, or other usable green fuels using solar energy under certain reactions [11–16]. Solar energy, being clean, stable, and renewable, serves a reliable source of energy for humanity [17,18]. However, due to the intermittent of sunlight, solar energy cannot be directly used as an energy source for daily life. Therefore, in recent years, the conversion of solar energy has gradually attracted extensive attention from all walks of life [19,20]. Artificial photosynthesis, also known as photocatalysis, has been developed as an effective strategy to convert solar energy into clean energy [21,22], which not only helps alleviate the energy crisis, but also mitigates the excessive emission of greenhouse gases [23–26].

Generally, the photocatalytic CO₂ reduction reaction (CO₂RR) involves three processes: photoabsorption and generation of photogenerated charge carriers, separation and transfer of photogenerated carriers, and reduction of CO₂ into other substances by active sites on the photocatalyst surface [27–29].

For an ideal photocatalyst to achieve higher efficiency in solar-energy conversion, the following features are essential: strong absorption of visible light, high carrier separation efficiency with a low recombination rate of photoelectron-hole pairs, and higher CO₂ reduction activity at the surface [30].

Semiconductor materials, with their effective light absorption and ingenious bandgaps, can meet these requirements by generating electron-hole pairs crucial for surface-initiated photocatalytic reactions [22,29,31]. Both all-inorganic and organic-inorganic hybrid semiconductor photocatalysts have garnered significant attention due to their excellent utilization of light and customizable structures. Among these, metal halides have emerged as one of the optimal options due to their unique properties, including strong absorption capability of visible light, tunable structures, negative conduction band positions, exceptional capability of charge transport, ease of synthesis, low fabrication costs, favorable for large-scale production, and low exciton binding energies. In fact, metal halide-based materials do have ideal development potential in this field and have been successfully developed as efficient photocatalysts [32–37].

However, such materials inevitably do present certain challenges. The separation and transport of photogenerated charges are severely hindered by significant energy defects, limiting their practical applications in terms of photocatalytic activity and cycling stability. In addition, the toxicity of lead in some of the best performing lead halide materials prevent their mass production. Apparently, there are many challenges in the current photocatalysis of CO₂, and corresponding solutions are urgently needed, which means that further improvement of photocatalysts is of great practical significance [38–40].

This review summarizes the latest progress in the use of perovskite and non-perovskite metal halides for photocatalytic CO₂RR from both organic-inorganic hybrids and all-inorganic perspectives. It begins with an introduction to the mechanism of photocatalytic CO₂ reduction, followed by an overview of common synthesis methods, including hydrothermal and anti-solvent precipitation. Afterwards, it provides a more detailed discussion on typical metal halide photocatalysts and relevant improvement strategies. Finally, it discusses future research challenges for metal halide photocatalysts, and offers positive and optimistic insights into their potential development prospects, especially in terms of improving stability, environmental friendliness, and product value. It is hoped that this review will provide a unique perspective on the development of metal halide photocatalysts and inspire innovative research in the fields of energy and material science.

2 Basic mechanism of photocatalytic CO₂ reduction

The process of photocatalytic CO₂RR begins when CO₂ molecules are adsorbed onto the catalyst surface, shown in Fig.1. Upon absorbing solar energy, photocatalysts generate photoexcited electron-hole pairs. This portion of energy must be at least equal to or greater than the bandgap value of the semiconductor (usually represented as E_g). Then, the excited photoelectrons migrate to the conduction band (CB), while the holes remain trapped in the valence band (VB). Further, these photogenerated electrons and holes move to the semiconductor surface, where the CO₂ molecules are adsorbed. Finally, CO₂RR occurs at catalytic active sites, where electrons are utilized. Concurrently, oxidation reactions mediated by holes are also stimulated, such as the decomposition of water to produce oxygen.

In general, both all-inorganic or organic-inorganic hybrid metal halide materials follow a similar mechanism in photocatalytic reduction of CO₂. However, there are distinct differences between the two. On the one hand, the orderly arrangement of organic components in hybrid materials may form efficient carrier transport channels, thereby enhancing carrier separation efficiency [41]. On the other hand, fully inorganic metal halide materials, such as CsPbBr₃, primarily activate CO₂ molecules through coordination with metal ions during the photoreduction process of CO₂. The reaction pathway is relatively straightforward, and the products are usually dominated by simple hydrocarbons like CO [42]. Organic cation engineering is expected to optimize the band edge and overall photocatalytic performance for CO₂ reduction of these materials. Some reported modifications, such as alkylation, can significantly increase the maximum occupied molecular orbital energy of cations, leading to increased product diversity [43].

Based on the above mechanism, it is clear that the catalytic efficiency of CO₂ reduction is determined by the adsorption of CO₂ molecules onto the catalyst surface. A high specific surface area (BET) is a primary requirement for effective catalysis. As is well known, CO₂ is an exceptionally self-stable molecule, with the decomposition of its carbon-oxygen double bond requiring 750 kJ/mol, or even as high as 803 kJ/mol. Hence, in the catalytic reduction of CO₂, energy higher than the activation energy barrier is needed to drive the reaction forward [30]. Furthermore, from the perspective of electrode potential, as shown in Tab.1 below, the redox potential for the conversion of CO₂ molecules into reaction intermediate *CO₂^– is approximately −1.90 eV, using a normal hydrogen electrode (NHE) as a reference. Therefore, it is extremely difficult to achieve the generation of *CO₂^– solely by transferring a single electron.

The presence of protons is crucial in facilitating the catalytic reduction process, ultimately leading to the generation of valuable chemicals containing one or more carbon atoms. However, the transfer mechanism and promoting effect of protons in photocatalytic CO₂ reduction still require further in-depth research. Unlike previous catalytic reactions, which are often conducted in acidic environments provided by HI, water offers mild and low-cost reaction conditions and serve as an ideal proton donor. In situ Fourier transform infrared (FT-IR) spectroscopy has been widely used to detect the intermediates produced during photocatalytic CO₂ reduction. Extensive characterization data shows that, after CO₂ molecules are activated by the catalyst, protons from water protonate them to form *COOH^–, which has a lower formation energy and serves as a key intermediate for conversion to CO.

This intermediate can be further protonated to produce *CHO, which is then converted into CH₄ with high fuel value through a series of steps, including dimerization [44]. Moreover, if *CO intermediates remain in the system and the reaction energy barrier for C-C coupling is effectively reduced, *CO and *CHO will play a crucial role in forming the key intermediate *OC-CHO, potentially leading to the production of C₂H₅OH or C₂H₆, both of which have higher industrial value [45]. Therefore, protons transferred to the catalytic active sites are pivotal for the formation of catalytic products.

For an ideal photocatalyst used in CO₂ reduction, the conduction band potential should be higher, and the valence band potential should be lower than the standard redox potentials of the reduction products, such as CO, CH₄, and HCHO. In other words, the band gap of the semiconductor must be wide enough to trigger photocatalytic redox reactions. However, if the bandgap is too large, the ability to utilize sunlight effectively will be limited. Recent studies have indicated that the optimal bandgap for semiconductors used in photocatalysis typically ranges from 1.8 to 2.8 eV [7,46].

Fig.2 illustrates some photocatalysts that meet these criteria and exhibit superior CO₂ reduction capabilities. On the other hand, an excessively small bandgap is also undesirable because it increases the likelihood of free electrons and holes recombining during their diffusion to the semiconductor surface. This recombination process causes the electrons and holes to return to their ground states, releasing energy. Generally, carrier recombination can occur through two mechanisms: radiative recombination and non-radiative recombination. It is clear that photogenerated electrons and holes would be consumed during the recombination process, which is detrimental to the progression of the photocatalytic reaction [47]. However, this issue can be mitigated in semiconductors with fewer intrinsic defects, allowing the reaction to favorably proceed toward the desired redox reactions [1].

The concept of “defect tolerance” refers to the property by which, even in materials with a high density of defects, the energy levels are generally not positioned within the band gap but are instead confined to the conduction or valence band [48–50]. This property enables metal halide perovskites (MHPs) to achieve a high carrier utilization rate, thereby enhancing their photocatalytic performance. For example, in the typical lead halide perovskite material APbX₃, the band gap is dominated by the antibonding orbitals of Pb and halogen atoms (Fig.3) [50]. The energy level of the defect is allowed to fall within either the conduction band or the valence band, not within the band gap itself. The term “shallow states” has been used to highlight this phenomenon, which has the potential to facilitate charge separation, making charge carrier utilization more efficient for such materials, and ultimately improving photocatalytic performance [49]. Over the past few years, MHPs have proven to be highly efficient photo-driven catalysts, emerging as novel cutting-edge materials in many solar driven applications [32,33,36,37,51–55].

3 Synthetic strategies of metal halide photocatalysts

Developing simple and effective synthesis strategies is an important prerequisite for broader application of metal halide photocatalysts. Several synthesis methods with flexible control of reaction conditions have been reported for the preparation of metal halide photocatalytic materials with different physicochemical properties [56–59]. These properties include factors such as composition, morphology, and more [60]. This section focuses on the widely used methods— hydrothermal, hot injection, ligand-assisted reprecipitation (LARP), and anti-solvent precipitation—for the synthesis of metal halide photocatalysts.

3.1 Hydrothermal method

The hydrothermal method is one of the most widely used techniques for synthesizing crystalline materials. Typically, the precursors are added as reactants with water serving as the reaction medium in a specially designed sealed vessel, such as an autoclave. By heating the reaction vessel, a high-temperature (100–1000 °C) and high-pressure (1.0–100 MPa) environment is created, allowing the insoluble powder reactants to dissolve and recrystallize (as shown in Fig.4(a)) The crystals prepared by the hydrothermal method usually has the characteristics of well-developed grains, small particle sizes, and uniform distribution. Moreover, this method allows the use of inexpensive raw materials and facilitates the formation of desired stoichiometry and crystal structures. These unique advantages have made the hydrothermal method widely applicable in active research areas such as material synthesis, chemical reactions, and chemical treatments.

For instance, in 2024, Yin et al. [61] conducted the first systematic study of the TJU-39(Pb) and TJU-40(Pb/TM, TM = Co²⁺/Ni²⁺) series of photocatalysts, which feature layered PbI₂ and three-dimensional (3D) transition metal intercalates. These photocatalysts demonstrated high efficiency in the solar-driven conversion of CO₂ to ethanol. In their synthesis, water and a small amount of perchloric acid were added to a Teflonlined autoclave reactor containing the precursors of PbI₂ and disodium iminodiacetate (imi), which was then heated in an oven at 160 °C for 48 h. After naturally cooling to room temperature, yellow plate-like crystals, TJU-39(Pb) were obtained by filtration and washed alternately with H₂O and DMF three times. The synthesis of TJU-40(Pb/TM) followed the same procedure as TJU-39(Pb), with the addition of a certain molar ratio of TM(OAc)₂. As-prepared resultants achieved efficient photocatalytic conversion of CO₂ to C₂H₅OH with the evolution rates of 24.9–31.4 μmol/(g·h), and selectivity exceeded 90%. As described above, the hydrothermal method is a practical technique for synthesizing metal halide photocatalytic crystals.

3.2 Hot-injection (H-I)

From the perspective of liquid-phase synthesis, the heat-injection (HI) method is widely used because it avoids the use of polar solvents and enables the synthesis of high-quality MHP materials through dispersed particles [1]. In this method, a completely dissolved precursor is rapidly injected into a hot solution containing raw materials such as organic ligands and high-boiling solvents (see Fig.4(b)). Conducting the process in a nitrogen atmosphere helps protect the synthesized material from degradation due to exposure to air, and the procedure also includes a degassing step. In 2024, Chen and colleagues reported the synthesis of a lanthanide (Ln³⁺)-doped layered double perovskite nanocrystal, Cs₄M(II)Sb₂Cl₁₂ (M(II): Cd or Mn), where Ln³⁺ can be Yb³⁺ or Er³⁺ [62], enabling efficient near-infrared (NIR) photoluminescence. Similarly, Han and other researchers reported that zero-dimensional (0D) Cs₃BiCl₆, Cs₃Bi₂Cl₉ and Cs₄MnBi₂Cl₁₂ nanosheets (NSs) can be synthesized using the H-I method, providing valuable insights into the application of bismuth-based halide perovskite materials in radiation detection [62]. By tuning different reaction temperatures, different volumes of crystals can be obtained, which is quite satisfactory. However, the thermal injection method has certain drawbacks, including the requirement for high temperatures, an inert atmosphere, and a low repetition rate, all of which limit its large-scale industrial application [63]. To address these issues, ligand-assisted reprecipitation (LARP) offers a more cost-effective solution, enabling the synthesis of high-quality metal halides at room temperature and pressure [64].

3.3 LARP

LARP synthesis of metal halide crystals involves using organic ligands in non-polar solvents (commonly hexane, toluene, etc.), while precursor salts such as lead halide, stannous halide, and others are dissolved in polar solvents (usually DMF, DMSO, etc.). When these two systems with different polarities are mixed, the solute becomes instantaneously supersaturated due to the polarity change, leading to spontaneous precipitation and crystallization. This promotes the nucleation and growth of MHP nanocrystals (Fig.4(c)). It is evident that LARP allows for precise control over crystal size at the nanometer level. Rtimi and his colleagues synthesized organic-inorganic halide perovskite MAPb_1–xZn_xBr_3–2xCl_2x nanocrystals using LARP [65], reducing the material’s toxicity by incorporating Zn²⁺. In 2023, Dhakal et al. [66] first synthesized orthogonal perovskite nanocrystals, CH(NH₂)₂SnI₃, with an average diameter of 7.7 nm and low toxicity via LARP. These nanocrystals demonstrated high stability for at least 265 days at room temperature and 20% relative humidity in an N₂ atmosphere. Given the simplicity of the required equipment, LARP shows great potential for large-scale production of metal halide nanocrystals, generating significant excitement in the field.

3.4 Anti-solvent precipitation (ASP)

The traditional H-I [67,68] and LARP methods [69–71] enable precise control over the size and morphology of MHPs [72]. Nonetheless, these methods are not suitable for large-scale synthesis of MHPs, limiting their industrial application. To address this limitation, the anti-solvent precipitation (ASP) method has been developed (Fig.4(d)) [1], offering advantages such as environmental friendliness, low cost, and simplicity [73–76].

Generally, the ASP method involves the following steps: the precursors are dissolved in an organic solvent (such as DMF or DMSO) to form an organic phase, followed by the addition of an anti-solvent, such as chlorobenzene or isopropanol. The solution is then heated and evaporated to achieve supersaturation, effectively separating the components between the solvent and the anti-solvent. Finally, MHP crystals precipitate as the solution cools. For example, Tang et al. successfully synthesized highly stable CsAgCl₂ crystals utilizing DMSO and isopropanol via the ASP method [77]. The obtained crystals possess excellent stability under air, heat, and light, along with outstanding performance in photocatalytic CO₂RR. The ASP method enhances the interfacial contact between MHPs and reaction substrates, thanks to its rapid growth kinetics that facilitate the reaction. As a result, MHPs are directly generated on the substrate surface. Other photocatalysts synthesized using the ASP approach include MAPbI₃ (MA⁺=methylammonium) [78], CsCuCl₃ [79], and Cs₂PdBr₆ [80]. While the ASP method is simple and easy to implement, it has inherent disadvantages, such as the production of micrometer-scale crystals and low crystallinity, which may have a negative impact on the photocatalytic performance of the materials.

4 Structural engineering strategies of metal halides for photocatalytic CO₂RR

With outstanding physical properties and specific chemical structure, metal halide-based photocatalysts have been successfully developed and increasingly utilized in the field of solar energy conversion into clean energy. These materials have shown promising applications in CO₂ photoreduction, photocatalytic H₂ evolution reaction (HER), photocatalytic degradation of organic pollutants, and the synthesis of organic compounds [81,82]. However, the separation of photogenerated electron-hole pairs competes with recombination processes, which limits the efficiency of these systems [83–85]. Furthermore, CO₂ reduction is a complex reaction that produces multiple products, with the yield of the target product being influenced by proton reduction processes. Therefore, it is crucial to understand the dynamics of photogenerated carriers and fully elucidate the mechanisms behind CO₂ reduction reactions.

4.1 All-inorganic metal halide perovskite photocatalysts

Among all inorganic metal halide perovskite photocatalysts, CsPbBr₃ (CPB)-based materials have been the most widely studied. Recently, CPB has become a research hotspot for photocatalytic applications due to its suitable bandgap, long electron-hole diffusion length, and excellent photoresponsivity [86,87]. CPB quantum dots (QDs) were first reported by Kuang et al. in 2017 for photocatalytic CO₂ conversion into chemical fuels (Fig.5(a)) [88]. The structure of CPB exhibits a typical octahedral shape, with Cs⁺ occupying the A-site and positioned at the vertices of the shared surface, as shown in Fig.5(b) [89]. Ethyl acetate (EA), with mild polarity, was selected as both the solvent and sacrificial agent, maximizing CO₂ dissolution while maintaining the stability of CPB. Under AM 1.5G simulated sunlight irradiation, CO₂ was reduced to CO and CH₄ at a rate of 23.7 mmol/(g·h), and the product selectivity reached to 99.3%.

When the semiconductor is processed into different nanostructures, both the surface and electronic structure of the material change, which in turn affects surface reactivity and band structure [90]. Meanwhile, adding an appropriate amount of water to the reaction system can reduce H₂ production and enhance the selectivity of CO₂ reduction [91]. However, excessive water intake negatively impacts the stability of PQDs. Subsequently, Sun and colleagues demonstrated that the bandgap, carrier lifetime, and electron-hole separation efficiency of CPB QDs (3–12 nm) are largely size-dependent [92]. If the size of QDs is too large, the surface area decreases, while if it is too small, aggregation occurs. The performance of CPB PQDs with four different sizes in the EA/H₂O environment for CO₂ reduction was compared, with the highest PQD yield observed at 8.5 nm after 8 h of sunlight irradiation (see Fig.5(c)).

Later, Chen et al. combined CPB perovskite NCs with a hybrid transition metal complex, Ni(tpy), and efficiently promoted the photocatalytic reduction of CO₂ to CO and CH₄ (Fig.5(d)) [93]. Under irradiation of simulated light from a 300 W Xenon lamp (λ > 400 nm), the total production of CO and CH₄ reached 431 μmol/(g·h). The catalytic activity for CO and CH₄ production under monochromatic light at 450 nm was 26 times higher than that of pristine CPB. High stability was confirmed by its performance over 16 h. It is evident that the introduction of Ni(tpy) provides more catalytic sites for CO₂ reduction, thus enhancing catalytic performance.

Unfortunately, the toxicity of Pb has always been a major factor limiting the wide application. Compared with the less stable Sn-based perovskites, Bi-based materials have proven to be the best substitutes. Bhosale et al. developed a series of non-toxic Bi-based halide perovskite materials, Rb₃Bi₂I₉ and Cs₃Bi₂I₉ [94]. These catalysts maintained 12-h working stability after UV irradiation for up to 7 days, as confirmed by XRD results. Under UV light (λ = 305 nm) for the same time, the production of CO followed the order: Cs₃Bi₂I₉ > Rb₃Bi₂I₉ > P25 TiO₂, while the catalytic activity toward CH₄ followed this order: Rb₃Bi₂I₉ > Cs₃Bi₂I₉ > P25 TiO₂ (Fig.5(e)). Two years later, Lu et al. reported pure antimony-based lead-free perovskite NCs, Cs₃Sb₂Br₉ [95]. Cs₃Sb₂Br₉ NCs showed a 10-fold enhancement in CO₂ reduction to CO activity, generating 510 μmol/g of CO in 4 h, compared to CsPbBr₃. This enhancement is attributed to the Sb active sites on the surface of Cs₃Sb₂Br₉, which efficiently generate *COOH and *CO intermediates, as supported by DFT calculations.

Different from CsPbBr₃-based materials, other MHPs containing multiple metal components have also been reported, such as the two-component perovskite with the general formula A₂B₁⁺B₂⁺X₆ [97,98]. Notably, the lead-free and highly crystalline Cs₂AgBiBr₆ (CABB) NCs [56,96,99], synthesized by Zhou et al. in 2018 using a common heat injection method, exhibit a high degree of stability, remaining intact for 3 weeks in mild or non-polar solvents. Methods to reduce ligand density by washing with absolute ethanol have been proposed to improve catalytic performance. XPS and Fourier Transform Infrared Spectrometer (FTIR) characterization results confirmed that the surface ligands could be completely washed away, and Thermogravimetric Analysis (TGA) results showed that the organic residues were also cleaned. The catalytic activity of washed CABB NCs for CO₂ reduction was greatly enhanced after 6 h, with increased CO and CH₄ production (Fig.5(f) and Fig.5(g)).

Up to present, increasing attentions have been paid to the development of lead-free biphasic halide perovskite photocatalysts, with a focus on both high stability and catalytic efficiency. Among these, Cs₂InSbCl₆ and Cs₂InBiCl₆ [100] have also been reported. Their suitable bandgap, low exciton binding energy, and high stability make them ideal candidates for the most promising lead-free halide perovskite photocatalysts, providing invaluable insights for the study of lead-free halides in photocatalysis.

4.2 All-inorganic metal halide non-perovskite photocatalysts

In all-inorganic metal halides, some non-perovskite compounds have attracted attention due to their excellent stability and photocatalytic performance. Currently, bismuth-based oxyhalide materials, known for their crystal tunability and compositional structure that significantly influence photocatalytic performance, have attracted extensive research [101,102]. For instance, Wang et al. [103] synthesized Bi₅O₇I nanosheets with porous structure from the parent material BiOIO₃ through high-temperature calcination, exhibiting enhanced photoresponsivity properties.

Unlike the aforementioned perovskite structure, it is evident that, as a non-perovskite photocatalyst, the inorganic components are no longer arranged in a regular octahedral form (see Fig.6(a)) [104]. The robust CO₂ reduction activity is attributed to the more efficient adsorption and activation of CO₂ molecules on the surface of Bi₅O₇I (E_ads = − 0.27 eV), as demonstrated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), DFT calculations, and other experimental analyses. The high photocurrent response of Bi₅O₇I indicates efficient charge separation and transfer, which is further supported by the significantly reduced photoluminescence (PL) intensity, suggesting a lower likelihood of electron-hole recombination.

Under visible light irradiation (λ > 420 nm), CO was the only product observed, with a yield of 10.67 μmol/g, nearly twice that of the parent material (Fig.6(b)). In a 12-h long-term illumination experiment, CO production showed a roughly linear growth trend, and no signs of deactivation were observed in the photocatalysts (Fig.6(c)), demonstrating good stability.

Unfortunately, the precise regulation of the state and number of active centers, along with the charge transfer process, remains a limiting factor for further applications due to the influence of the contact interface [106]. To develop catalysts with high-density active sites, Li et al. employed a strain-induced strategy to polarize the dual surface interface. By using strain engineering, they successfully assembled a Cu porphyrin-based atomic layer (PML-Cu) with Bi₁₂O₁₇Br₂ (BOB), resulting in the formation of PML-Cu/Bi₁₂O₁₇Br₂ (PBOB) (Fig.6(d)), triggering a multi-step polarization process at the surface interface [105]. The work function (Φ), calculated via ultraviolet photoelectron spectroscopy (UPS), indicates that the electrons in BOB tend to flow toward PML-Cu, creating an internal electric field (IEF) [107]. This IEF accelerates the polarization of the interface in PBOB, enhancing the electron transfer between the layers. The local polarization field disrupts charge symmetry, promoting the effective separation of photogenerated electrons and holes [108,109].

XPS valence band spectra revealed that the VB of BOB and PBOB were 1.21 and 1.18 eV, respectively. Combined with UV-vis spectral data, the CB of the two were inferred to be −1.02 and −0.94 eV, respectively (Fig.6(e)). The band structure implies that the interface-polarized PBOB can be used for CO₂ photoreduction to CO, as validated by photocatalytic CO₂ reduction performance tests. In these experiments, the only carbon containing product, CO, was precipitated in PBOB with a yield of 584.3 μmol/g after 5 h (Fig.6(f)), achieving performance that was 7.83 and 21.01 times higher than that of BOB and PML-Cu, respectively. The catalytic activity of PBOB remained at 90.60% after 5 cycles, demonstrating that the as-polarized samples exhibit robust stability.

It is worth noting that H₂O₂ was detected alongside CO₂ reduction as a highly practical photocatalytic oxidation product. After the reaction, the content of H₂O₂ can reach 63.80 μmol/g in the solution. This suggests that the induced surface interface dual polarization strategy can effectively address the issues of limited active sites and slow charge transfer in photocatalysts, highlighting its potential for designing photocatalysts with concentrated active sites.

In a similar study, Gong et al. [110] used oxygen vacancies (OVs) and bismuth clusters to assist Bi₁₂O₁₇Cl₂ nanosheets in achieving efficient and selective CO₂ photoreduction. The hydrothermal method was employed to prepare OV-rich and bismuth-rich ultrathin nanosheets, with OV-poor and bismuth-poor Bi₁₂O₁₇Cl₂ samples prepared for comparison. The synthesized multifunctional OV-Bi-Bi₁₂O₁₇Cl₂ exhibited a CO release rate of 64.3 μmo/(g·h), which was 15 times higher than that of the OV-poor sample. In addition, this performance is significantly improved by 28 times compared to bulk Bi₁₂O₁₇Cl₂ (2.3 μmol/(g·h)), as shown in Fig.7(a). Remarkably, similar CO yields were maintained over four consecutive cycles without any structural or morphological changes, demonstrating the excellent photocatalytic activity and stability of OV-Bi- Bi₁₂O₁₇Cl₂ (Fig.7(b)). It is suggested that OV can impart a negative charge to Bi₁₂O₁₇Cl₂, creating electron trapping sites that promote the adsorption and activation of CO₂. Additionally, the Bi clusters act as photohole receptors, and the synergistic effect of the OV and Bi clusters enhances carrier transfer, improving charge separation efficiency and offering potential advantages for subsequent photoreduction.

In a more recent study, Yang and colleagues anchored Co atoms onto ultra-thin BiOCl single-crystal nanosheets using hydrothermal and sintering engineering techniques. XRD, Raman spectroscopy, and TEM characterizations confirmed the successful synthesis of these materials [111]. The addition of polyvinylpyrrolidone (PVP) inhibited the stacking of [Cl⁻Bi-O₂-BiCl] units between layers, preserving the ultrathin nanosheet morphology. The introduction of Co atoms enhances electron delocalization around Bi-O and Co-O bonds, promoting electron transfer. Simultaneously, the conduction band of the material exhibits a noticeable downward shift, improving electron mobility. These findings are supported by the density of states (DOSs) calculations. Compared to the typical two-dimensional indirect semiconductor BiOCl, the as-synthesized 2Co-BiOCl enhances the average CO generation rate by approximately 13 times, reaching 183.9 μmol/(g·h) (Fig.7(c)). A series of control experiments-conducted without light, water, or the photocatalyst, confirmed that the CO generated originates from the photocatalytic reduction of CO₂ and water, demonstrating that this process is indeed photo-driven. To highlight the uniqueness and advantages of Co-BiOCl materials, the performance of 2Co-BiOCl was compared to that of other reported bismuth-based catalysts. Remarkably, the catalytic performance of 2Co-BiOCl ranks among the best, positioning it as an efficient and stable candidate for carbon reduction applications (Fig.7(d)).

4.3 Organic-inorganic hybrid metal halide perovskite photocatalysts

In recent years, metal halide perovskites have become a focal point in photocatalytic CO₂RR research, offering unique advantages such as excellent carrier mobility and tunable bandgaps [112–116]. When combined with metal-organic frameworks (MOFs), their CO₂ reduction efficiency has been significantly enhanced [117]. However, the widespread use of metal halide perovskite is still limited, primarily due to their low conversion efficiency and stability [118]. To address these challenges, one promising strategy involves introducing organic compounds to replace the ions at A sites in the perovskite structure. Que et al. reported that colloidal FAPbBr₃ QDs, which exhibit a lower carrier recombination rate and high thermal, optical, and water stability, can effectively overcome these difficulties (Fig.8(a)) [119]. The high crystallinity cubic samples (Fig.8(b)) were prepared by the H-I method. FAPbBr₃ crystallizes in a cubic system, with six halogen anions located at the corners and center of the cube, respectively. These anions interconnect with Pb ions at the center of the unit cell, forming a well-structured octahedral three-dimensional skeleton, characteristic feature perovskite catalyst. Compared to traditional CsPbBr₃ QDs, FAPbBr₃ QDs have a broader absorption spectrum and retain 40% of their fluorescence intensity after 520 min of irradiation in a deionized water (DI)/ethyl acetate (EA) environment. Under 300 W Xenon arc lamp irradiation in DI/EA, FAPbBr₃ QDs achieved a CO yield of 181.25 μmol/(g·h), with an electron consumption (R_electron) of 504.44 μmol/(g·h), accompanied by a small amount of CH₄. The CO₂ selectivity exceeded 97%, effectively inhibiting the hydrogen evolution reaction (HER) in this process.

The first top-down synthesis of MA₃Bi₂I₉ by Diau and colleagues provides a benchmark strategy for preparing non-toxic and stable photocatalysts for the photoreduction of CO₂ to high-value fuels like CH₄ [94]. The isolated [Bi₂I₉]^3- ion in MA₃Bi₂I₉ supports its zero-dimensional structure, which effectively preserves the octahedral configuration typical of perovskite, thanks to the formation of the face-sharing Bi-I bonds [120]. The photoreduction stability of the synthesized sample was confirmed by maintaining the XRD pattern adequately after one week at 70% RH, followed by 12 h of continuous ultraviolet irradiation. UPS data provides direct evidence that MA₃Bi₂I₉ nanocrystals (NCs) are suitable for CO₂RR, with their energy levels between −5.29 and −2.89 eV, aligning with the reduction potentials of CO₂/CO and CO₂/CH₄ at −3.97 and −4.26 eV, respectively (Fig.8(c)). The photoreduction experiment demonstrated that MA₃Bi₂I₉ NCs could generate CH₄ at a yield of approximately 10.4 μmol/g, while CO was produced at a yield of 7.2 μmol/g, as detected by gas chromatography-mass spectrometry (GC-MS). Mechanistic studies indicate that A-site cations and crystal structure significantly affect the catalytic activity of photocatalysts. Notably, the yield of both products in the organic-inorganic hybrid sample is lower than that of the all-inorganic photocatalyst prepared using the same method (Fig.8(d)). This may be attributed to the stabilization of holes in MA₃Bi₂I₉ by Bi⁴⁺, •CH₂NH₃⁺ radical cations, and oxygen anions, which reduces the efficiency of hole transfer to the desired oxidation channel. The quenching of the electron paramagnetic resonance (EPR) signal in the MA-based material diminishes, weakening the catalytic performance of MA₃Bi₂I₉ compared to the other two inorganic cation NC catalysts.

Ultimately, bismuth-based photocatalysts offer key advantages such as a wide bandgap, high binding energy, and non-toxicity, making them promising candidates for various optoelectronic applications.

Improving the stability of organic-inorganic hybrid lead halide perovskite materials in polar solvents is crucial for enhancing CO₂ conversion in practical applications. One effective approach involves designing MOFs to encapsulate perovskites, providing protection similar to a cage around the metal halide perovskites (MHPs). Specifically, the composite photocatalyst MAPbI₃@PCN-221(Fe_x) (where x = 0 – 1), was successfully fabricated by sequentially depositing the classical lead halide perovskite MAPbI₃ QDs within an iron porphyrin MOF, as reported by Lu et al. [116]. The structure of MAPbI₃ is illustrated in Fig.9(a) [121]. The results show that when x = 0.2, CO and CH₄ were produced by MAPbI₃@PCN-221(Fe_0.2) under 300 W Xe-lamp irradiation, achieving a high total yield of 1559 μmol/g, which is 38 times higher than that from the original PCN-221(Fe_0.2), which was only 41 μmol/g. A continuous reaction lasting 80 h demonstrated a significant improvement in stability of the composite, as shown in Fig.9(b) and Fig.9(c). The reaction, conducted in an EA/DI system where water served as the electron source, primarily produced CO for both the Fe-free components, PCN-221, and MAPbI₃@PCN-221. The high photocatalytic electron transfer rate is attributed to the strong interaction between MAPbI₃ QDs and the Fe catalytic sites within the MOF. The number of these catalytic sites plays a crucial role in the formation of CH₄. Furthermore, the high activity of the metal centers is linked to the porous and tunable crystal structure of the MOFs, which provides an excellent specific surface area for enhanced photocatalytic performance.

4.4 Organic-inorganic hybrid metal halide non-perovskite photocatalysts

Although inorganic organic hybrid perovskite systems show amazing application prospects, it is important to note that organic compounds with strong coordination abilities cover a wide range of categories. Therefore, using organic compounds as ligands has become a widely accepted strategy for synthesizing structurally novel catalysts without losing stability. Inorganic-organic hybrid non-perovskite compounds have emerged and demonstrated superior stability. For example, replacing MA⁺ with hydrophobic quaternary ammonium cations, such as tetra-methyl ammonium [122,123] or tetra-butyl ammonium [124,125], which are adsorbed on the surface of as-prepared materials, has been reported to inhibit the absorption of water by the lattice. This modification enables photocatalysts to remain stable for over 30 days at a relative humidity of 90 ± 5% without any loss in photovoltaic performance [126]. In addition, the introduction of organic ligands often improves the energy level structure of all-inorganic materials, enhancing their light absorption capacity and adsorption of CO₂, making them more suitable for photocatalytic CO₂RR.

In 2022, Wu et al. [127] prepared brown plate-like monoclinic crystals, TMOF-10-NH₂(Br/I), using PbI₂ or PbBr₂ and 2-aminoterephthalic acid as the ligand via a hydrothermal method. This approach achieved a high yield of approximately 80% and excellent phase purity. The following discussion will focus on TMOF-10-NH₂ (I) for further analysis. Single crystal analysis revealed that, unlike the FAPbBr₃ perovskite mentioned earlier, TMOF-10-NH₂ exhibits an equal-network 3D coordination framework with 1D pores (Fig.10(a)). The halogen-bridged, rod-like secondary structural units are formed by the planar inorganic components, extending along the a-axis in a sawtooth-like shape. Structurally, non-perovskite catalysts like TMOF-10-NH₂ differ significantly from the perovskite materials described earlier. The thermostability of TMOF-10-NH₂ can be maintained up to 200 °C, and it also shows high moisture stability in the range of 50% to 90% relative humidity (RH). Furthermore, even after 48 h of continuous illumination (300 W Xe lamp, 24 W/cm²), its strong photostability remains intact. Stability tests, along with calculations of CO₂ and water vapor uptake in the material’s pores, confirmed that TMOF-10-NH₂ provides a robust catalytic platform for photocatalytic CO₂RR in the presence of water vapor. DFT calculations indicate a lower CO₂ adsorption energy E_Ads of − 0.63 eV on the surface of TMOF-10-NH₂, which is favorable compared to most reported photocatalysts [107]. This suggests that TMOF-10-NH₂ has superior CO₂ adsorption activity, which may be attributed to the strong interaction induced between oxygen atoms in *CO₂ and Pb²⁺ in TMOF-10-NH₂. The band structure plays a crucial role in enabling photocatalytic reactions. Valence band X-ray photoelectron spectroscopy (VB-XPS) revealed that the energy level of the valence band maximum (VBM) of TMOF-10-NH₂(I) is 1.78 eV, while the conduction band minimum (CBM) was estimated to be −1.11 eV based on Mott-Schottky analysis. Therefore, the CBM of TMOF-10-NH₂ is more negative than the redox potential of CO₂/CO (−0.48 V versus NHE, pH = 7) or CO₂/CH₄, while the VBM is more favorable than the redox potentials of O₂/H₂O (+0.82 V versus NHE, pH = 7) or H₂O₂/H₂O (+1.35 V versus NHE, pH = 7) (Fig.10(b)). These results provide thermodynamic support for the realization of CO₂ photoreduction and H₂O oxidation on TMOF-10-NH₂. Additionally, photophysical studies, including surface photovoltage (SPV), transient absorption spectroscopy (TA), and Hall effect measurements, confirm the high carrier mobility and long carrier diffusion length in TMOF-10-NH₂.

The outstanding photocatalytic performance of TMOF-10-NH₂ is demonstrated in several tests. Under AM 1.5G simulated sunlight for 4 h, TMOF-10-NH₂ consistently emitted CO at an average rate of 78 μmol/(h·g), outperforming the majority of MOFs in CO₂ photo-reduction by water vapor. In a continuous cycling experiment, TMOF-10-NH₂ successfully catalyzed the continuous production of CO for 24 h, showing little change in its structure (as confirmed by PXRD, FT-IR, and SEM characterization). These results validate the effective catalytic activity of TMOF-10-NH₂ in cyclic reactions and further highlight its high stability and excellent durability. The high CO release rate of TMOF-10-NH₂(I) indicates that its metal-iodine secondary building unit (SBUs) has better photocatalytic performance compared to conventional metal-oxo MOFs and other organolead halide materials (Fig.10(c)) [128,129]. Notably, there is no direct contact between the aqueous medium and the photocatalyst, and the absence of Pb²⁺ leaching enhances the environmental friendliness of these materials.

Intriguingly, when Ru nanoparticles (NPs) were successfully loaded onto the surface of TMOF-10-NH₂(I) crystals, the efficiency of photocatalytic CO₂RR was nearly doubled. The optimal loading amount was found to be 1.58% (mass fraction), as verified by inductively coupled plasma optical emission spectroscopy (ICP-OES). At this loading, the CO release rate reached 154 μmol/(g·h) (Fig.10(d)), surpassing the vast majority of MOFs and/or lead halide hybrid catalysts, with no metal leaching or morphological changes observed. This enhancement can be attributed to the effective suppression of radiative electron-hole pair recombination in TMOF-10-NH₂(I) by the introduction of Ru NPs. Overall, this study provides valuable insights for the development of MOFs photocatalysts with halide-bridged SBUs, making small molecule activation and efficient photocarrier transport more promising in future research.

Building on this study, the photocatalysts TJU-31 and TJU-32 for the photocatalytic reduction of CO₂ to ethanol were reported several months later [45]. Both photocatalysts were synthesized using lead iodide as the precursor, with TJU-31 employing glutarate as the ligand and TJU-32 using iminodiacetate as the ligand (Fig.11(a)). By precisely controlling the feeding ratio and reaction conditions, yellow plate-like and block-shaped crystals were obtained for TJU-31 and TJU-32, respectively. The phase purity of both materials was confirmed by the excellent agreement between their PXRD patterns and the simulated single-crystal data. Both TJU-31 and TJU-32 demonstrated excellent thermal stability, structural robustness and high photostability. This was confirmed by the absence of structural collapse after 24 h of continuous illumination at 270 °C, exposure to a wide pH range (4 to 10), boiling water conditions, and 90% RH. This stability is attributed to the formation of coordination grids, which contrasts with the ion binding nature of organolead iodide perovskites that are easily displaced by proton molecules [130]. Notably, in TJU-32, the ‒NH sites provided by-the ligands coordinate with additional Pb²⁺ centers, linking adjacent organic layers into a 3D sublattice. This transition from a two-dimensional (2D) to a three-dimensional (3D) structure creates asymmetric multi-metal sites, significantly reducing the energy barrier for C‒C coupling during photocatalytic CO₂ reduction. As a result, this structure facilitates the efficient generation of C₂ product, particularly ethanol.

Ultraviolet photoelectron spectroscopy (UPS) and UV-vis data indicate that TJU-32 has a wide bandgap, with the VBM determined to be 1.98 V and the CBM at −0.64 V (Fig.11(b)). Therefore, simultaneous CO₂ photoreduction and H₂O photooxidation are thermodynamically feasible.

After AM 1.5G simulated sunlight irradiation, TJU-32 not only steadily evolved ethanol at a rate of 17.6 μmol/(h·g), but also released CO and CH₄ at rates of 3.9 and 0.4 μmol/(h·g), respectively. The deposition of metal NPs as cocatalysts is a common strategy to enhance charge separation and photocatalytic activity. Similarly, loading various concentrations of Rh onto the crystal surface of TJU-32 using RhCl₃ solutions significantly enhanced charge separation and photocatalytic activity. Among the tested samples, Rh_0.11@TJU-32 showed the highest ethanol evolution rate of 50.5 μmol/(h·g), which is three times higher than that of the parent TJU-32 (Fig.11(c)). The corresponding selectivity for ethanol also increased from 80.4% to 89.4%, making it one of the highest among the developed CO₂ to C₂H₅OH photocatalysts reported to date. Notably, the results of five consecutive experiments showed no remarkable decrease in ethanol production (Fig.11(d)), confirming the long-term photostability of Rh_0.11@TJU-32 under continuous irradiation for 20 h.

Several low-toxic or non-toxic metal cations have been reported as potential B-site components, exhibiting similar electronic structures and ionic radii to Pb²⁺, thus showing promise as replacements. Recently, coordination polymers containing Cu⁺ have become a hot topic in photocatalysis research. Bai and colleagues successfully used a dark red octahedral crystal, named NJU-Bai61 (Fig.12(a)), to produce CH₄ through the photoreduction of CO₂ [131]. This study marks the first-time application of the Cu(I)_xX_yL_z (X = Cl, Br, or I; L = organic ligands containing N, P, or S) coordination polymer for photocatalytic CO₂ reduction. In this system, Cu₄I₄ clusters adsorb photogenerated electrons and transfer them to Cu₃OI(CO₂)₃ clusters, which act as active sites for CO₂ adsorption and reduction. Theoretical calculations show that this process provides enough electrons for the reduction of CO₂ to CH₄. Experimental data indicate that after 4 h of illumination, NJU-Bai61 efficiently generates CH₄ with a yield of 15.75 μmol/(h·g) (Fig.12(b)), exhibiting a high selectivity of 72.8% in aqueous solution, with only small amounts of CO and H₂ as by-products. Cyclic experiments showed no significant change in CH₄ generation activity (Fig.12(c)), and XRD characterization confirmed that the crystal maintains robust stability before and after the photocatalytic reaction. This research provides significant insights into the development of lead-free catalysts for CO₂ photoreduction and the potential of new MOF systems.

The incorporation of metal ions into catalysts derived from certain MOF materials is considered an effective strategy for CO₂ photoreduction applications. Fei et al. successfully integrated a manganese halide complex into highly robust MOFs, using DMF/TEOA as a medium, and employed visible light to reduce CO₂ to formats [132], a class of high-value products used as preservatives and antibacterial agents in livestock feed, as well as in the leather and textile industries. The turnover number of UiO-67-Mn(bpy)(CO)₃Br reached approximately 50 after 4 h of visible light irradiation at 470 nm, with 96% selectivity. Extended reaction times of up to 18 h resulted in a turnover number of about 110 (Fig.12(d)), surpassing many noble metal-containing MOF catalysts. However, it is important to note that the lifespan of the prepared material cannot be solely defined by this turnover value, despite the photocatalytic reaction time being 18 h in the figure. The significant activity enhancement can be attributed to the sufficiently large pores of UiO-67, which facilitate electron transfer between Ru(II) photosensitizers and Mn complexes within the MOF. The framework backbone provides active sites for electron separation, improving the overall stability of the catalyst while preventing the dimerization of the Mn complex.

The aforementioned organic-inorganic hybrid metal halide materials offer a new reference point for coordination chemistry, providing a foundation for researchers to rationally design highly efficient materials for CO₂ photoreduction.

5 Summarization and prospectives

Photocatalysts for solar energy conversion and storage have garnered extensive research interest in recent years. Among these, metal halides have shown great development potential due to their excellent properties, sparking in-depth studies. This review introduces the common methods for synthesizing photocatalysts based on the fundamental mechanisms of photocatalytic CO₂ reduction, with a particular focus on hydrothermal synthesis, hot-injection, LARP, and anti-solvent precipitation. Moreover, it presents the latest progress in CO₂RR photocatalysts and exciting research in solar energy-driven applications, focusing on two main directions: organic-inorganic hybridization and all-inorganic systems, with perovskite and non-perovskite materials as subcategories. Tab.2 summarizes some of the materials reported for photocatalytic CO₂RR in recent years.

In addition, various strategies to enhance photocatalytic performance are explored, including surface engineering (e.g., supported cocatalysts), tuning reaction parameters (e.g., solvent types, nanocrystal size), heterojunction engineering (e.g., integrating photocatalysts with MOFs), and composite engineering (e.g., halogen replacement, energy band adjustment, cation or anion regulation). While photocatalysts have made notable breakthroughs in this field, their widespread application is still in the early stages. To develop more efficient and compliant photocatalysts, several critical challenges must be addressed:

(1) Toxicity of Pb-based materials: For a long time, the superior performance of Pb-based photocatalytic materials has made them the most extensively studied. However, the high toxicity of Pb to both the environment and organisms remains the primary factor limiting their large-scale application [133]. Lead can leach from perovskites into plants, entering the human food chain [134], posing a serious threat to human health [135]. Therefore, the primary consideration for the long-term utilization of metal halide photocatalysts is replacing Pb at the B sites [11,136–138]. Several low- or non-toxic metal ions, such as Bi³⁺, Cu⁺, Sn²⁺, and Ag⁺, have been explored as potential substitutes for lead. These lead-free environmentally friendly photocatalysts not only eliminate Pb-related toxicity, but also exhibit higher catalytic activity and chemical stability [139–143]. Nevertheless, their performance still lags behind that of Pb-based materials, highlighting a significant opportunity for improvement in the development of efficient lead-free photocatalysts.

(2) Stability under harsh conditions: Metal halide catalysts often struggle to maintain structural stability under high temperature, humidity, and light conditions, primarily due to the more active chemical bonds at the particle surface and weaker interactions between ligands and metals. Additionally, the ionic bonding nature of metal halide materials makes them more prone to decomposition. To improve catalyst stability, it is essential to use organic ligands that bind more firmly to metal atoms during synthesis or to create protective passivation layers on the material’s surface. For example, encapsulating material molecules within a waterproof framework can help ensure long-term stability while maintaining or even enhancing catalytic performance [117].

(3) Production of high-value hydrocarbon products: To date, the majority of metal halide photocatalysts have thus far been limited to reducing CO₂ to C₁ products, such as CO, or HCOOH, through the two-electron (2e^–) reduction process. The ability to synthesize more advanced hydrocarbon energy products containing multiple carbon atoms through 4e^–, 6e^–, or 8e^– reduction remains a long-awaited goal. This highlights the importance of gaining a deeper understanding of the reaction mechanisms in photocatalytic CO₂ reduction to produce high-value organic compounds.

In summary, metal halides hold great potential for solar energy conversion to green energy. Further research and development of robust metal halide photocatalytic materials are essential to advancing solar-driven technologies and achieving a low-carbon future. It is hoped that this review provides valuable insights into recent advancements in photocatalysis, contributing significantly to the scientific research and application of the photovoltaic industry.

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